Microfabrication technologies have recently been developed in order to attain high integration and high performance of a semiconductor integrated circuit (hereinafter referred to as LSI). A chemical mechanical polishing (hereinafter referred to as CMP) method is one of the technologies, which is often used for flattening interlayer dielectrics, metal plug formation, and embedded wiring formation in a multilayer wiring formation step. The technology is disclosed in Patent Publication 1, for example.
Also, in order to attain the high performance of LSI, use of copper or a copper alloy as a conductive material for a wiring material has recently been tried. However, it is difficult to perform microfabrication of copper or copper alloy by employing the dry etching method which has often been employed for conventional aluminum alloy wiring formation.
Accordingly, a so-called damascene method, in which a thin film of copper or copper alloy is deposited on an insulation film on which grooves are formed, and the thin film except for the groove portions is removed by CMP to form an embedded wiring, has predominantly been employed. The technology is disclosed in Patent Publication 2, for example.
In an ordinary method of CMP of a metal, which is employed for polishing a conductive material such as copper and a copper alloy used as a wiring metal, a polishing cloth (pad) is attached to a circular polishing plate (platen); a metal film formed on a substrate is pressed to a surface of the polishing pad while the surface of the polishing pad is impregnated with a CMP polishing liquid; the polishing platen is rotated in a state where a predetermined pressure (hereinafter referred to as polishing pressure) is applied to the metal film from a reverse side of the polishing pad; and the metal film on a convex portion is removed by relative mechanical abrasion between the CMP polishing liquid and the convex portion of the metal film.
The CMP polishing liquid used in the metal CMP is generally formed of an oxidizing agent and an abrasive grain, and an oxidized-metal dissolving agent and a protective film-forming agent are further added as required. Conductive material surfaces are oxidized by the oxidizing agent, and the generated oxidized conductive material film is ground by the abrasive grain, which is the basic mechanism.
Since the oxidized conductive material film on the surface of a concave portion is scarcely in contact with the polishing pad, the effect of polishing by the abrasive grain does not exerted on the oxidized conductive material film, and the substrate surface is flattened by removal of the conductive material on the convex portion along with a progress of CMP. Details are disclosed in Non-Patent Publication 1, for example.
It is known that addition of the oxidized-metal dissolving agent is effective as one of the methods for increasing a speed of the polishing by CMP. It is understood that the grinding effect of the abrasive grain is enhanced when grains of the oxidized conductive material polished off by the abrasive grain are dissolved (hereinafter referred to as etching) into the CMP polishing liquid.
Though the CMP polishing speed is improved by the addition of oxidized-metal dissolving agent, the conductive material surface is further oxidized by the oxidizing agent when the conductive material surface is exposed by the etching on the oxidized conductive material in the convex portion. The etching on the conductive material in the convex portion progresses when the oxidization is repeated. Therefore, the flattening effect is impaired by a phenomenon that a central portion of the surface of the conductive material which is embedded after the polishing is depressed like a dish (hereinafter referred to as dishing).
Addition of a protective film-forming agent for the purpose of preventing the dishing is known. The protective film-forming agent is capable of forming a protective film on the oxidized film on the conductive material surface to thereby prevent the oxidized film from being dissolved into the CMP polishing liquid. It is desirable that the protective film is easily ground with the use of the abrasive grain and does not reduce the CMP polishing speed.
In order to suppress the dishing and corrosion during polishing of the conductive material and to form a high-reliability LSI wiring, a method of using an oxidized-metal dissolving agent selected from aminoacetic acids and amidosulfuric acids and a CMP polishing liquid containing BAT (benzotriazole) as the protective film-forming agent has been proposed. The technology is described in Patent Publication 3, for example.
Meanwhile, as shown in FIG. 1(a), a barrier conductive film 2 (hereinafter refereed to as barrier film) for preventing copper diffusion into interlayer dielectrics 1 and for improving adhesion is formed below a conductive material 3 formed of a wiring metal such as copper and a copper alloy. Therefore, it is necessary to eliminate the exposed barrier film 2 by CMP on portions other than the wiring portion in which the conductive material is embedded. However, since a conductor used for the barrier film 2 has higher hardness as compared to the conductive material, it is difficult to attain a satisfactory polishing speed even when a polishing material for conductive material is used in combination, and the flatness is often deteriorated.
Therefore, a two-step polishing method has been studied, in which the process is divided into “first polishing step” of polishing the conductive material 3 from the state shown in FIG. 1(a) to the state shown in FIG. 1(b) and “second polishing step” of polishing the barrier film 2 from the state shown in FIG. 1(b) to the state shown in FIG. 1(c). Also, in order to improve the flatness of the surface after termination of the polishing, apart of a convex portion of the interlayer dielectrics 1 is generally polished in the second polishing step, which is called “overpolishing”.